feat(#1458): Renderable Codec Protocol

[dimitri-yatsenko]

Summary

Implements T3.2 of the 2.3 release plan, against the spec in datajoint-docs#188.

A runtime-checkable Protocol that codecs opt into by implementing `render_spark(self, decoded, *, key=None) -> Any`. Consumers (e.g., a Databricks silver-layer publish pipeline) detect support via `isinstance(codec, Renderable)`.

Closes #1458. Supersedes #1457. Slated for DataJoint 2.3.

What's added

Component	File
`Renderable` Protocol declaration	`src/datajoint/rendering.py` (new, ~85 lines including docstrings)
Top-level re-export as `dj.Renderable`	`src/datajoint/init.py`
9 unit tests	`tests/unit/test_rendering.py` (new)

The Protocol surface is intentionally tiny — a single method. The bulk of `rendering.py` is docstring (module-level rationale + class docstring documenting allowed return-value shapes with worked codec examples).

Tests

• 9/9 pass on the new file.
• Coverage: opt-in detection, non-opt-in rejection, top-level re-export, `@runtime_checkable` guarantee, built-in `<blob@>` and `<hash@>` codecs are NOT Renderable (per spec contract), invocation pass-through, `key` kwarg acceptance, subclass opt-in behavior.

What's NOT in this PR (out of scope per spec)

• Specific renderable codec implementations. `<float_array@>`, `<image_2d@>`, `<labeled_struct@>`, `<timeseries@>`, etc. ship downstream as plugins. They register via existing codec auto-registration and opt in by implementing `render_spark()`.
• Silver-layer publish pipeline. Lives in `datajoint-databricks`. This PR provides the eligibility-check contract; the pipeline consumes it.
• No `decode_spark` (reverse direction). Delta consumers query rendered columns directly via Spark SQL.
• No `BINARY` fallback. Codecs either implement `Renderable` or remain non-eligible — no automatic blob → bytes-passthrough.

Sequencing

Independent of the provenance trinity (T2.2.a–c, in flight) — no code overlap. Lands in parallel.

Once this PR merges, the matching docs spec PR (datajoint-docs#188) flips from draft to ready for review.

Test plan

• 9/9 new unit tests pass
• CI green (lint, test matrix, unit-tests)
• `isinstance(codec, dj.Renderable)` returns True for a plugin codec that adds the method (smoke-test against a sample plugin once one exists)